Construct broker-owned filesystems

[Weidong Cui (wdcui)]

This PR constructs the broker-authoritative filesystem on Linux and Windows as a writable in-memory layer over an optional tar archive with broker devices mounted at /dev, exposes the archive only through the explicit --fs-initial-files broker option, and leaves existing runner-local filesystems unchanged.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type BrokerCoreBuilder is no longer UnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_userland/src/builder.rs:77
  type BrokerCoreBuilder is no longer RefUnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_userland/src/builder.rs:77